High resolution optical waveform shaping device having phase shift compensation associated with optical intensity modulation

ABSTRACT

It is an object of the present invention to provide an optical waveform shaping device of high resolution. 
     The above-mentioned problem is solved by an optical waveform shaping device ( 10 ) comprising a branching filter ( 11 ) for dividing the light beam from a light source into light beams of each frequency, a condensing part ( 12 ) for condensing a plurality of light beams divided by the branching filter ( 11 ), a polarization separation means ( 13 ) for adjusting the polarizing planes of the light beams having passed through the condensing part ( 12 ), and a spatial light modulator ( 14 ) having a phase modulation part and an intensity modulation part where the light beams having passed through the polarizing plate ( 13 ) are incident.

TECHNICAL FIELD

The present invention relates to an optical waveform shaping device andso on.

BACKGROUND ART

The waveform of an optical signal transmitted in an optical transmissionsystem degrades by ASE noise, non-linear characteristics of an opticalfiber, etc., resulting in degradation of transmission quality. In such acase, the degraded waveform of the optical signal is recovered by anoptical waveform shaping device for shaping the waveform an opticalsignal. Furthermore, an optical waveform shaping device is used in anobservation device using a femtosecond laser etc., for example, as it isimportant to shape a laser waveform.

For example, JP-A 2001-42274 discloses an optical waveform shapingdevice with a spatial light modulator for phase modulation and a spatialphase modulator for intensity modulation. However, the optical waveformshaping device disclosed in the publication uses two modulators eachhaving a grass substrate, which inevitably leads to the expansion of thediameter of a beam. Thus, there is a problem of low resolution.

-   Patent document 1: JP-A 2003-90926-   Patent document 2: JP-A 2002-131710

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is an object of the present invention to provide an optical waveformshaping device of high resolution.

It is an object of the present invention to provide an optical waveformshaping device of high resolution which is capable of phase shiftcompensation associated with optical intensity modulation.

It is an object of the present invention to provide an optical waveformshaping device with a passband generally rectangular in shape. It is anobject of the present invention to provide an optical waveform shapingdevice which is capable of ultrafast optical clock generation of theterahertz order.

It is an object of the present invention to provide a band variableoptical waveform shaping device which is capable of miniaturization.

Means for Solving Problems

The present invention is basically based on a knowledge that the use ofa spatial light modulator having a phase modulation part and anintensity modulation part can provide an optical waveform shaping deviceof high resolution, resulting in a new applied technology includingultrafast optical clock generation of the terahertz order.

That is, the first aspect of the present invention relates to an opticalwaveform shaping device (10) comprising: a polarization separator (1)for polarizing/separating the light beam from a light source; a ½wavelength plate (2) for joining the polarization planes of a firstlightwave and a second lightwave polarized/separated by the polarizationseparator (1); a polarization beam splitter (3) where the light beamshaving passed through the ½ wavelength plate (2) are incident; a Faradayrotator (4) for rotating in a predetermined amount the polarizationplanes of the first lightwave and the second lightwave having passedthrough the polarization beam splitter (3); a first collimator (5) wherethe lightwave having passed through the Faraday rotator (4) is incident;a second collimator (6) where the lightwave having passed through theFaraday rotator (4) is incident; a 2-axis polarization-preserving fiber(7) where the lightwaves from the first collimator and the secondcollimator are incident; a third collimator (8) where light beam havingpassed through the 2-axis polarization-preserving fiber (7); a branchingfilter (11) for branching the light beam from the third collimator (8)into the light beams of each frequency; a condensing lens (12) forcondensing the plurality of light beams branched by the branching filter(11); a polarization separation means (13) for adjusting thepolarization planes of the light beams having passed through thecondensing lens (12); a spatial light modulator (14) having a phasemodulation part and an intensity modulation part where the light beamshaving passed through the polarization separation means (13) areincident, the phase modulation part and the intensity modulation parteach having a plurality of liquid crystal cells in a line or in a matrixexisting in the corresponding spatial positions, the orientation ofliquid crystals of the phase modulation part being parallel to thepolarization plane adjusted by the polarization separation means (13),the orientation of liquid crystal of the intensity modulation part being45 degrees offset from the orientation of liquid crystals of the phasemodulation part; a prism-type folded reflector (15) where the lightbeams having passed through the liquid crystal spatial phase modulationand liquid crystal spatial intensity modulation part (14) are incident;a ½ wavelength plate (16) for adjusting the polarization planes of thelightwaves output from the polarization beam splitter (3) after havingpassed through the folded reflector (15); and a forth collimator (17)where the light beams having passed through the ½ wavelength plate (16)are incident, wherein the light beam from the third collimator (8) isfrequency separated and is dispersed spatially by the branching filter(11), wherein the spatially dispersed and frequency separated lightbeams are condensed by the condensing lens (12), wherein thepolarization planes of the condensed light beams are adjusted by thepolarization separation means (13), wherein the light beams with thepolarization planes adjusted are subjected to either or both ofseparately controlled phase modulation and intensity modulation by thespatial light modulator (14), wherein the light beams are folded by thefolded reflector (15), wherein the light beams are condensed through thecondensing lens (12), wherein the frequency separated light beams aremultiplexed by the branching filter (11), wherein the lightwave derivedfrom the first lightwave is incident on the Faraday rotator (4) throughthe second collimator (6), wherein the lightwave derived from the secondlightwave is incident on the Faraday rotator (4) through the firstcollimator (5), wherein the traveling direction of the lightwave derivedfrom the first lightwave and the lightwave derived from the secondlightwave having passed through the Faraday rotator (4) are adjusted bythe polarization beam splitter (3), wherein the polarization planes ofthe two lightwaves with the traveling direction adjusted are adjusted bythe ½ wavelength plate (16) so that the polarization planes areorthogonal to each other, and wherein the lightwaves with thepolarization planes adjusted are output through the forth collimator(17).

As demonstrated in the embodiment, the optical waveform shaping deviceis an optical waveform shaping device with an extremely highspecification.

The preferred embodiment of the first aspect of the present invention isthe optical waveform shaping device wherein the plurality of liquidcrystal cells of the spatial light modulator (14) constitute one channelwith two cells or three cells depending on the diameter of input light.One channel means a unit of cell receiving one input light.Specifically, the plurality of liquid crystal cells of the spatial lightmodulator (14) comprise a lattice pitch of 10-40 μm, for example. Thus,setting the number of cells in view of wavelength dependency ofdispersion characteristic by a branching filter improves the resolutionof wavelength space.

The second aspect of the present invention relates to an opticalwaveform shaping device (10) comprising: a polarization separator (1)for polarizing/separating the light beam from a light source; abranching filter (11) for branching a first lightwave and a secondlightwave separated by the polarization separator (1) into the lightbeams of each frequency; a condensing part (12) for condensing theplurality of light beams branched by the branching filter (11); apolarization separation means (13) for adjusting the polarization planesof the light beam having passed through the condensing part (12); aspatial light modulator (14) having a phase modulation part and anintensity modulation part where the light beams having passed throughthe polarization separation means (13); and a prism-type foldedreflector (15) where the light beams having passed through the spatiallight modulator (14) having the liquid crystal spatial phase modulationand liquid crystal spatial intensity modulation part (14) are incident.

Thus, alternating the two optical paths back and forth by a both-sidestelecentric system for the polarized/separated two optical paths caneliminate the influence on the output by phase change in thepolarization/separation portion.

The preferred embodiment of the second aspect of the present inventionrelates to the optical waveform shaping device (10) wherein the firstlightwave and the second lightwave reach the branching filter (11)through a first axis and a second axis, respectively, of a 2-axispolarization-preserving fiber (7) and also the first lightwave and thesecond lightwave folded through the reflector (15) are output throughthe second axis and the first axis, respectively, of the 2-axispolarization-preserving fiber (7).

Spatial filtering by combining the degradation of the beam qualityderived from the polarization separation control with a 2-axis PM canremove an irregular change from the intensity distribution of a laserbeam. This can prevent the expansion of the condensing diameter of thebeam in the optical intensity control part and the optical phase controlpart of the spatial light modulator (14), resulting in the improvementof resolution.

The preferred embodiment of the second aspect of the present inventionis the optical waveform shaping device wherein the plurality of liquidcrystal cells of the spatial light modulator (14) constitute one channelwith two cells or three cells depending on the diameter of input light.One channel means a unit of cell receiving one input light.Specifically, the plurality of liquid crystal cells of the spatial lightmodulator (14) comprises a lattice pitch of 10-40 μm, for example. Thus,setting the number of cells in view of wavelength dependency ofdispersion characteristic by a branching filter improves the resolutionof wavelength space.

In the optical waveform shaping device (10) according to the thirdaspect of the present invention, the polarization separation means (13)comprises a polarization beam splitter (61) and an optical system forguiding a first light and a second light separated by the polarizationbeam splitter (61) to the branching filter (11). This configurationallows effective utilization of the light beam returning back from thespatial light modulator (14), polarized/separated and discarded.

In the preferred embodiment of the third aspect of the presentinvention, the third collimator (8) further comprises a polarizationmodule (51) located at the end face of the 2-axispolarization-preserving fiber (7). And the polarization module comprisesa first optical system (53, 52, 57) and a second optical system (56, 54,55). And the first optical system (53, 52, 57) controls the lighttraveling to the spatial light modulator (14) and the first lightseparated by the polarization separation means (13) after having passedthrough the spatial light modulator (14). On the other hand, the secondoptical system (56, 54, 55) controls the second light separated by thepolarization separation means (13).

This can improve the insertion loss of 1.5 dB derived from thepolarization separation.

Effect of the Invention

The optical waveform shaping device of the present invention uses onespatial light modulator (14) having a phase modulation part and anintensity modulation part, and the phase modulation part and theintensity modulation part have a glass substrate in common, whichprevents the expansion of the diameter of a beam, thereby providing highresolution.

The optical waveform shaping device of the present invention furtherfeedbacks the phase shift associated with optical intensity modulationto the control voltage of liquid crystals, or adjusts the orientation ofa polarizer and the liquid crystals in the phase modulation part of thespatial light modulator to compensate the phase shift associated withintensity modulation, which compensates the phase shift associated withthe optical intensity modulation.

The optical waveform shaping device of the present invention can providean optical waveform shaping device with a passband generally rectangularin shape as was confirmed in an actual device.

The optical waveform shaping device of the present invention can be usedas a band variable optical waveform shaping device as the passbands ofadjacent bands form continuous passbands.

The optical waveform shaping device of the present invention can beminiaturized precisely as optical elements can be omitted and theinfluences such as dispersion can be compensated in case it is areflection type.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 are conceptual diagrams showing an example of a configuration ofan optical waveform shaping device of the present invention. FIG. 1( a)is a top view and FIG. 1( b) is a side view. As shown in FIG. 1, theoptical waveform shaping device of the present invention, that is, thefirst aspect of the present invention comprises a polarization separator(1) for polarizing/separating light beam from a light source, a ½wavelength plate (2) for joining the polarization planes of a firstlightwave and a second lightwave polarized/separated by the polarizationseparator (1), a polarization beam splitter (3) where the light beamshaving passed through the ½ wavelength plate (2) are incident, a Faradayrotator (4) for rotating in a predetermined amount the polarizationplanes of the first lightwave and the second lightwave having passedthrough the polarization beam splitter (3), a first collimator (5) wherethe lightwave having passed through the Faraday rotator (4) is incident,a second collimator (6) where the lightwave having passed through theFaraday rotator (4) is incident, a 2-axis polarization-preserving fiber(2-PMF (7)) where the lightwaves from the first collimator and thesecond collimator are incident, a third collimator (8) where light beamhaving passed through the 2-axis polarization-preserving fiber (7), abranching filter (11) for branching the light beam from the thirdcollimator (8) into the light beams of each frequency, a condensing lens(12) for condensing the plurality of light beams branched by thebranching filter (11), a polarization separation means (13) foradjusting the polarization planes of the light beams having passedthrough the condensing lens (12), a spatial light modulator (14) havinga phase modulation part and an intensity modulation part where the lightbeams having passed through the polarization separation means (13) areincident, the phase modulation part and the intensity modulation parteach having a plurality of liquid crystal cells in a line or in a matrixexisting in the corresponding spatial positions, the orientation ofliquid crystals of the phase modulation part being parallel to thepolarization plane adjusted by the polarization separation means (13),the orientation of liquid crystal of the intensity modulation part being45 degrees offset from the orientation of liquid crystals of the phasemodulation part, a prism-type folded reflector (15) where the lightbeams having passed through the liquid crystal spatial phase modulationand liquid crystal spatial intensity modulation part (14) are incident,a ½ wavelength plate (16) for adjusting the polarization planes of thelightwaves output from the polarization beam splitter (3) after havingpassed through the folded reflector (15), and a forth collimator (17)where the light beams having passed through the ½ wavelength plate (16).

The light from a light source is incident on a polarization separator(1) through a single mode fiber etc, for example. The polarizationseparator (1) polarizes/separates the incident light. In this case, thepolarization planes of the two lights are orthogonal to each other, forexample.

The first lightwave and the second lightwave polarized/separated by thepolarization separator (1) are incident on a ½ wavelength plate (2). The½ wavelength plate (2) rotates either of the polarization planes of thefirst lightwave and the second lightwave polarized/separated by thepolarization separator (1), for example, so that the polarization planesare joined. That is, the ½ wavelength plate (2) turns the polarizationplanes of the two lightwaves in the same direction.

The lightwaves having passed through the ½ wavelength plate (2) areincident on a polarization beam splitter (3). The lightwaves havingpassed through the polarization beam splitter (3) are incident on aFaraday rotator (4). The Faraday rotator (4) rotates in a predeterminedamount the polarization planes of the first lightwave and the secondlightwave having passed through the polarization beam splitter.

The first lightwave having passed through the Faraday rotator (4) isincident on a first collimator (5), while the second lightwave havingpassed through the Faraday rotator (4) is incident on a secondcollimator (6). Then, the lightwaves from the first collimator (5) andthe second collimator (6) are incident on a 2-axispolarization-preserving fiber (7). The lightwaves are output to the2-axis polarization-preserving fiber (7) with the polarization planesmaintained. Furthermore, spatial filtering is given by the 2-axispolarization-preserving fiber (7), resulting in the resolutionimprovement.

The light beams having passed through the 2-axis polarization-preservingfiber (7) are incident on a third collimator (8). The light beam fromthe third collimator (8) is frequency separated and is dispersedspatially by the branching filter (11). The frequency separated andspatially dispersed light beams after having passed through thebranching filter (11) are condensed by the condensing lens (12).

The polarization planes of the condensed light beams are adjusted by thepolarization separation means (13). The light beams with thepolarization planes adjusted are subjected to either or both of theseparately controlled phase modulation and intensity modulation by thespatial light modulator (14).

After the modulation, the light beams are folded by a folded reflector(15). Then, the folded light beams are condensed through the condensinglens (12) and are multiplexed by the branching filter (11).

The lightwave derived from the first lightwave is incident on theFaraday rotator (4) through the second collimator (6), while thelightwave derived from the second lightwave is incident on the Faradayrotator (4) through the first collimator (5). And, the opticalpolarization planes are rotated in a predetermined amount by the Faradayrotator (4). The polarization beam splitter (3) adjusts the travelingdirection of the lightwave derived from the first lightwave and thelightwave derived from the second lightwave having passed through theFaraday rotator (4).

A ½ wavelength plate (16) adjusts the polarization planes of the twolightwaves having passed through the polarization beam splitter (3) sothat the polarization planes are orthogonal to each other, and thelightwaves with the polarization planes adjusted are output through aforth collimator (17).

The branching filter (11) is an element for branching the light from alight source into light beams of each frequency. As a branching filter,a grating, a prism, or a high-dispersion element such as a grism may beused. Alternatively, an AWG may be used. As a light source, white lightor light containing a plurality of wavelengths of light, for example,may be used. Alternatively, pulsed light with a wavelength ofapproximately 1550 nm may be used. As for the light beam from a lightsource, the polarizing plane may be adjusted with a polarizationadjuster, a polarizing plate, etc. Furthermore, the light beam from alight source may be polarized and separated into two kinds of lightbeams having mutually-perpendicular polarizing planes, for example.

The condensing lens (12) serves as a condensing part for condensing aplurality of light beams condensed by the branching filter (11). Awell-known condensing lens can preferably be used as a condensing lens.The condensing lens (12) may be provided in the spatial position wherethe light beams spatially dispersed by the grating (11) can be condensedand can be guided to a predetermined cells of the spatial lightmodulator (14).

The polarization separation means (13) is an optical element foradjusting the polarization plane of the light beam having passed throughthe condensing lens (12). As a polarizing plate, a well-known polarizingplate or a polarizer can preferably be used. An interference film typepolarizer is more preferable as the polarizing plate. The use of such aninterference film type polarizer indicates the use of a polarizer with alarge diameter, which leads to improvement of convenience.

The spatial light modulator (14) where the light beams having passedthrough the polarization separation means (13) is incident has a phasemodulation part and an intensity modulation part each having a pluralityof liquid crystal cells in a line or in a matrix existing in thecorresponding spatial positions. For example, in the above Patentdocument 1, a spatial phase modulation part and a spatial intensitymodulation part are separated from each other. On the other hand, in thepresent invention, a phase modulation part and an intensity modulationpart are joined together and are arranged on a glass substrate. This canreduce the number of glass substrates used in a spatial light modulatorto one, which can prevent the expansion of the diameter of a beam,thereby providing high resolution. In order to control unnecessaryreflection, the phase modulation part and the intensity modulation partare preferably joined together so that each refractive index is matched.A plurality of liquid crystal cells in a line means a plurality ofliquid crystal cells arranged in a straight line, while a plurality ofliquid crystal cells in a matrix means a plurality of liquid crystalcells arranged in good order vertically and horizontally. The pluralityof liquid crystal cells arranged in a straight line is more preferable.And the orientation of liquid crystals of the phase modulation part isparallel to the polarization plane adjusted with the polarizing plate(3), for example, while the orientation of liquid crystals of theintensity modulation part is offset from the orientation of liquidcrystals of the phase modulation part. A specific offset in theorientation of the intensity modulation part is preferably in the rangeof 30 degrees-60 degrees, more preferably 40 degrees-50 degrees, mostpreferably 45 degrees. The liquid crystal spatial phase modulation partmay exist in the front (the side of the polarization separation means)of the liquid crystal spatial intensity modulation part, or the liquidcrystal spatial intensity modulation part may exist in the front.

FIG. 2 is a conceptual diagram of a spatial light modulator having aphase modulation part and an intensity modulation part. As shown in FIG.2, a spatial light modulator (14) comprises an intensity modulation part(22) having a plurality of liquid crystal cells (21) formed in a line orin a matrix, and a phase modulation part (24) having a plurality ofliquid crystal cells (23) corresponding to the liquid crystal cells (21)of the intensity modulation part. The liquid crystal cells (21) of theintensity modulation part (22) and the liquid crystal cells (23) of thephase modulation part (24) each comprise liquid crystal substances aswell as electrodes holding the liquid crystal substances therebetween.This electrode may be a transparent electrode or a metal electrodeexisting anywhere in the circumference of the cells. An example of aspecific configuration is such that two liquid crystal elements with alattice pitch of 10 μm-40 μm are joined together and are mounted on aglass substrate. The lattice pitch is a factor determining the width ofeach cell. As shown in FIG. 2, a gap may be provided between theadjacent liquid crystal cells (21, 23).

FIG. 3 is a conceptual diagram showing the orientation of an intensitymodulation part and a phase modulation part. As shown in FIG. 3, theorientation of the intensity modulation part is 45 degrees offset fromthat of the phase modulation part, for example. In order for phasemodulation and intensity modulation to be performed with these liquidcrystal elements, the polarization plane by the polarizing plate may beparallel to the orientation of liquid crystals of the phase modulationpart, and the orientation of liquid crystals of the intensity modulationpart may be 45 degrees offset from the polarization plane by thepolarizing plate. The offset angle in the intensity modulation part maybe any value except 0 degree. However, 45 degrees is preferable from aviewpoint of controlling intensity easily.

FIG. 4 are conceptual diagrams for explaining polarization control,intensity control, and phase control. FIG. 4( a) is a diagram showingthe intensity modulation and the phase modulation of the presentinvention. FIG. 4( b) is a diagram showing the optical phase shift whenonly the intensity modulation is performed. As shown in FIG. 4( b), whenonly the intensity modulation is performed, intensity is adjusted withan intensity modulator, and linear polarization is changed to circularpolarization. Then, circular polarization is changed back to linearpolarization with a polarizer. The intensity modulation is performed inthis way. However, as shown in FIG. 4( b), though the optical phase isback to liner polarization, the phase state is changed. On the otherhand, as shown in FIG. 4( a), in a system having both intensitymodulation and phase modulation, the phase modulation compensates thephase shift by the intensity modulation, which allows the phase ofoutput light to be matched with the phase of input light.

A smaller condensing diameter of the liquid crystal cells on thecondensing lens side is more preferable as it reduces the width of theobtained bandpass. From this viewpoint, the condensing diameter may bein the range of 20 μm-80 μm, preferably 30 μm-70 μm. And the size of theliquid crystal cells may be 10 μm-40 μm, preferably 15 μm-30 μm, or itmay be 15 μm-25 μm. The use of such microscopic cells enables thepassbands of a 10 GHz interval. Furthermore, as a wavelength becomeslarger, the condensing diameter becomes larger, and thus one light beamon the short wavelength side may be received by two liquid crystal cellswhile one light beam on the long wavelength side may be received bythree liquid crystal cells. The condensing diameter refers to thediameter of the light beam derived from the image formed on the liquidcrystal cells by a plurality of light beams condensed by the condensinglens.

A folded reflector (15) is an optical element where the light beamshaving passed through a spatial light modulator (14) having a phasemodulation part and an intensity modulation part are incident and shiftthe traveling direction. A well-known optical element such as a mirrorand a prism can preferably be used as the folded reflector. Thisadoption of reflection type enables the overlapped use of opticalelements and also enables compensating for the influences of dispersionetc., resulting in precise miniaturization of an optical waveformshaping device.

FIG. 5 is a diagram showing an example of an optical waveform shapingdevice which uses a prism as a folded reflector. As shown in FIG. 5, theuse of a prism (26) can ensure vertical or horizontal optical paths. Asa result, two kinds of light beams can follow symmetrical optical paths,thereby equalizing the influences such as a noise derived from theoptical paths.

Lightwaves are dispersed spatially for each frequency by a grating (11).The spatially-dispersed lightwaves are condensed with a condensing lens(12), and are incident on a spatial light modulator (14) through apolarizing plate (13), where the lightwaves are subjected toseparately-controlled phase modulation and intensity modulation. At thespatial light modulator (14), the lightwaves are incident on differentcells for each dispersed frequency. FIG. 6 are figures showing a beamincident on cells. FIG. 6( a) shows a light beam on the short wavelengthside incident on cells, while FIG. 6( b) shows a light beam on the longwavelength side incident on cells. The cell size is, for example, a 20μm interval, and the light beam on the short wavelength side may beincident on two cells, while the light beam on the long wavelength sidemay be incident on three cells. Thus, the light beams in real time canbe resolved into frequencies and can be expanded in real space.Furthermore, the number of cells to a certain beam can be adjustedaccording to the wavelength dispersion characteristic of a grating.Light beams are folded by a folded reflector (15). The folded lightbeams are condensed with the condensing lens (12), and thespatially-dispersed light beams are multiplexed by a branching filter(11). Thus, each spatially-dispersed light beam is multiplexed with itsphase and intensity adjusted.

The invention has been explained in the above with reference to areflection type optical waveform shaping device. However, the opticalwaveform shaping device of the present invention may be a transmissiontype. Specifically, a transmission type optical waveform shaping devicemay comprise a grating, a first condensing lens, a polarizing plate, aspatial light modulator having a phase modulation part and an intensitymodulation part, a second condensing lens, and an optical multiplexer.The same lens as the first condensing lens may be used as the secondcondensing lens. The same thing as the grating may be used as theoptical multiplexer.

FIG. 7 are figures showing an optical waveform shaping device of thepresent invention capable of phase shift compensation. FIG. 7( a) showsan example using an existing driver, while FIG. 7( b) shows an exampleperforming DIO direct control. In FIG. 7( a), a control device such as aPC is connected with a voltage control part (32), and the voltagecontrol part has a SLM driver 1 and a SLM driver 2 which control thedriving voltage applied to a spatial phase modulation part and a spatialintensity modulation part. On the other hand, in the case of DIO directcontrol as shown in FIG. 7( b), the driving voltage applied to thespatial phase modulation part and the spatial intensity modulation partis directly controlled in accordance with the instructions from thecontrol device. As shown in FIG. 7, this optical waveform shaping devicecomprises a spatial light modulator (14), a detection part (31), acontrol device (32), and a voltage adjustment part (33). And the controldevice outputs control signals for instructing the voltage adjustmentpart based on the phase shift detected by the detection part. On theother hand, the voltage adjustment part outputs predetermined voltage tothe electrode of each cell according to the received controlled signals.Thus, according to the optical waveform shaping device of the presentinvention, the phase shift accompanied with intensity modulation can becompensated.

The detection part (31) is an element for detecting output light fromthe optical waveform shaping device when intensity modulation isperformed by an intensity modulation part (22). As the detection part, awell-known detection device such as a photodiode can arbitrarily beemployed. The detection part (31) is preferably provided within thechassis of the optical waveform shaping device. Furthermore, thedetection part preferably monitors the controlled variable relating toboth optical intensity and optical phase.

The control device (32) is a device for receiving information relatingto the phase shift of each frequency detected by the detection part (31)and controlling the voltage applied to the electrode of each liquidcrystal cell (23) of the phase modulation part (24). Specifically, acomputer serves as the control device. The control device may beprovided as a unit with the optical waveform shaping device or may beprovided externally. In terms of downsizing the device, the controldevice is preferably provided within the chassis of the optical waveformshaping device. When the detection part monitors the controlled variablerelating to both optical intensity and optical phase, such control ispreferably performed by a closed loop so that optical intensity andoptical phase come closer to a set value following a comparison ofmeasured optical intensity and optical phase with the set value. Thiscontrol of optical intensity and optical phase can increase thestability of the device.

The voltage adjustment part (33) outputs the voltage applied to theelectrodes of each liquid crystal cell (23) of the phase modulation part(24) to each liquid crystal cell of the phase modulator (24) inaccordance with the control instructions from the control device (32).Furthermore, instead of feedback control, the orientation direction of apolarizer and the phase modulation part (24) may be adjusted so thatphase modulation can be performed to compensate the phase shift byintensity modulation. That is, it is a preferable mode of the presentinvention to use the orientation direction of liquid crystals of apolarizing plane of the polarizer and the phase modulation part (24) sothat phase modulation can be performed to compensate the phase shift byintensity modulation. The intensity modulation part may have a similarconfiguration so that the intensity variation accompanied with phasemodulation.

The optical waveform shaping device of the present invention can be usedas a light source for WDM etc. Furthermore, the optical waveform shapingdevice of the present invention can be used as an optical transmissiondevice for EDFA etc.

FIG. 16 are figures for explaining the optical shaping device accordingto the third aspect of the present invention. FIG. 16A is a figure forexplaining the first difference with the optical waveform shaping deviceshown in FIG. 1. FIG. 16B is a figure showing a polarization module. Inthe figures, numeral 51 indicates a polarization module, numerals 52-55indicate polarization beam splitters, and numeral 56 and 57 indicate aλ/2 wavelength plate. Numerals 52 and/or 54 may be a mirror. In FIG.16A, the polarization module is provided at the end face of a 2-axispolarization-preserving fiber (7) existing within a third collimator(8). The polarization module may be bonded to the end face of the 2-axispolarization-preserving fiber (7) with an optical adhesive. Thisconfiguration provides the third collimator capable ofpolarization/separation.

FIG. 17 is a figure for explaining the optical waveform shaping deviceaccording to the third aspect of the present invention. As shown in FIG.17, the optical waveform shaping device does not adopt a mere polarizingplate or polarizer but adopts the configuration shown in FIG. 17 as apolarization separation means (13). Numerals 61 and 62 indicatepolarization beam splitters, numeral 64 indicates a λ/2 wavelengthplate, and numeral 65 indicates a double-sided AR plate. Numeral 62 maybe a mirror.

Hereinafter, the operation of the optical waveform shaping deviceaccording to the third aspect of the present invention will bedescribed. Basic operations are the same as those of the opticalwaveform shaping device according to the first aspect of the presentinvention. The operations of the spatial light modulator shown in FIG.17 will be described. The waveform shaping device according to the firstaspect of the present invention uses a polarizing plate as apolarization separation means, for example. In this case, a part of thepolarized/separated light beams will be discarded. On the other hand,the optical waveform shaping device according to the third aspect of thepresent invention adopts the configuration shown in FIG. 17, and thusthe discarded light beams can be used effectively. That is, as shown inFIG. 17, the polarization separation means (13) has a polarization beamsplitter (61) as well as an optical system for guiding the first lightand the second light separated by the polarization beam splitter (61) toa branching filter (11). As the first light and the second light arepolarized/separated by the polarization beam splitter (61), theirpolarization planes are different. This configuration allows effectiveutilization of the light returned from the spatial light modulator andthen polarized/separated and discarded. And one example of such anoptical system comprises a polarization beam splitter (61), apolarization beam splitter (62), a λ/2 wavelength plate (64) and adouble-sided AR plate (65). That is, the light beams returned by areflector (15) after having passed through the spatial light modulator(14) are incident on the polarization beam splitter (61). And the lightbeams incident on the polarization beam splitter (61) are separated tothe first light and the second light by the polarizing planes. One ofthe light separated travels to a branching filter (11) as in the opticalwaveform shaping device according to the first aspect of the presentinvention. This is called “first light” herein. On the other hand, theother light polarized/separated (this is called “second light” herein)is, after polarized/separated or adjusted its traveling direction by thepolarization beam splitter (62), incident on the λ/2 wavelength plate(64). Then, the polarizing plane is adjusted by the λ/2 wavelength plate(64). This allows effective utilization of the remaining lightpolarized/separated. The optical system for guiding the second light tothe branching filter may be designed arbitrarily (e.g., using a mirrorinstead of the polarization beam splitter (62)).

As for the polarization module shown in FIG. 16 preferably has a firstoptical system (53, 52, 57) and a second optical system (56, 54, 55).The first optical system (53, 52, 57) is an optical system through whichthe light traveling to a spatial light modulator (14) and the firstlight polarized/separated through the spatial light modulator (14) pass.The second optical system (56, 54, 55) is an optical system for thesecond light polarized/separated through the spatial light modulator(14). The first optical system (53, 52, 57) comprises a polarizationbeam splitter (53) where the light output from the end face of a 2-axispolarization-preserving fiber (7) is incident, an optical element (52)where one of the light beams polarized/separated by the polarizationbeam splitter (53), and a λ/2 wavelength plate (57) where the lightbeams having passed through the optical element (52). Examples of theoptical element (52) include a polarization beam splitter, a polarizingplate, a polarizer, and a mirror. In case the first lightpolarized/separated through the spatial light modulator (14) returns tothe polarization module (51), the light passes through the λ/2wavelength plate (57), the optical element (52), and the polarizationbeam splitter (53) back to the 2-axis polarization-preserving fiber (7).The second optical system (56, 54, 55) has a polarization beam splitter(55), and an optical element (54) and a λ/2 wavelength plate (56).Examples of the optical element (54) include a polarization beamsplitter, a polarizing plate, a polarizer, and a mirror. The secondlight polarized/separated through the spatial light modulator (14)passes the λ/2 wavelength plate (56), the optical element (54), and thepolarization beam splitter (55) in this order back to the 2-axispolarization-preserving fiber (7). That is, the polarization plane ofthe second light polarized/separated through the spatial light modulator(14) is adjusted by the λ/2 wavelength plate (56), polarized/separatedby the polarization beam splitters (mirrors) (54, 55) and adjusted itstraveling direction, returns to the 2-axis polarization-preserving fiber(7). This allows bidirectional operations. Furthermore, the provision ofa number of optical separation means allows multichannelization.

Embodiment 1

FIG. 8 is a comprehensive diagram of the optical waveform shaping deviceaccording to Embodiment 1. FIG. 9 are schematic diagrams of the opticalsystem according to Embodiment 1. FIG. 9( a) is a top view, while FIG.9( b) is a side view. In the figures, PBS indicates a polarizing beamsplitter, FR indicates a Faraday rotator, SMF indicates a single modefiber, and 2-PMF indicates a 2-axis polarization-preserving fiber. Asshown in FIG. 9, this optical system comprises a 2-axispolarization-preserving fiber (2-PMF), a collimating lens with adiameter of 15 cm and a focal length of 6 cm, a grating having a surfacecenter position at 6 cm from the collimating lens, a condensing lens(f15 cm) positioned at 15 cm from the surface center position of thegrating, a polarizing plate, a liquid crystal spatial intensityadjustment part, a liquid crystal spatial phase modulation part, and afolded reflector. The position of the folded reflector (prism) was setat 15 cm from the condensing lens. The width of the control part of eachliquid cell was set 17 μm, and the size of the gap part was set 3 μm.That is, one cell size was 20 μm. The distance between the collimatinglens and the grating and the distance between the collimating lens andthe condensing lens were calculated by doing simulation as shown in FIG.10.

Phases fluctuate sensitively to the changes such as tension ortemperature of a fiber. As this embodiment employs the aboveconfiguration, when the two optical paths of fibers are replaced witheach other, the outward path and the return path will receive the samephase shift in total. As a result, though fibers etc. lack phasestability, an optical waveform shaping device with high phase stabilitycan be provided.

FIG. 11 are figures showing an example of a liquid crystal spatial lightmodulator. FIG. 11( a) through FIG. 11( c) are figures showing anoverview of the actually manufactured spatial light modulator. Thespatial optical modulator shown in FIG. 11( a) used a glass substratewith a width of 65 mm×48 mm and a thickness of 0.5 mm. The size of aliquid crystal cell gap located between glass substrates having a commonelectrode and a pattern electrode respectively was set 8 μm. The widthof a liquid crystal lattice was 14×14 mm, and it was installed near thecenter of the glass substrates. The pitch of the liquid crystal latticewas 20 μm (specifically, the control area was 17 μm and the gap was 3μm). As for the orientation of liquid crystals, the orientationdirection was set 45 degrees in case of intensity control, and it wasset 0 degree in case of phase control. The liquid crystal spatial lightmodulator for intensity control and the liquid crystal spatial lightmodulator for phase control were prepared separately.

In FIG. 11( b), the glass substrate with a width of 65× up to 30 mm anda thickness of 0.3 mm was used. The size of a liquid crystal cell gaplocated between glass substrates having a common electrode and a patternelectrode respectively was set 8 μm. The liquid crystal lattice with asize of 10 mm× up to 5 mm was used. In FIG. 11( b), the liquid crystalspatial light modulator was put to either the left or the right of theglass substrate. This intentional arrangement of the liquid crystallight modulator away from the center enabled easier preparation. As apitch of the liquid crystal lattice, the following three patterns weremanufactured:

(i) Pitch of liquid crystal lattice: 20 μm (control area: 17 μm, gap: 3μm)

(ii) Pitch of liquid crystal lattice: 20 μm (control area: 18 μm, gap: 2μm)

(iii) Pitch of liquid crystal lattice: 10 μm (control area: 8 μm, gap: 2μm)

As for the orientation of liquid crystals, the orientation direction wasset 45 degrees in case of intensity control, and it was set 0 degree incase of phase control. The liquid crystal spatial light modulator forintensity control and the liquid crystal spatial light modulator forphase control were prepared separately. When connecting the two liquidcrystal elements, a marker for positioning the lattices was provided.Furthermore, control ICs were collectively arranged on one side of theglass substrate.

In the spatial light modulator as shown in FIG. 11( c), a liquid crystalspatial light modulator for intensity control and a liquid crystalspatial light modulator for phase control are provided on each of thesurface and the rear surface respectively of the glass substrate havinga pattern electrode. These were prepared with the portion of thelattices aligned. As shown in FIG. 11( c), this liquid crystal spatiallight modulator has the configuration where the glass substrate havingthe pattern electrode is put between two substrates having a commonelectrode. The size of a liquid crystal cell gap located between glasssubstrates each having a common electrode and a pattern electrode wasset 8 μm. The liquid crystal lattice with a size of up to 20 mm×up to 5mm was used. In FIG. 11( c), the liquid crystal spatial light modulatorwas put to either the left or the right of the glass substrate. Thisintentional arrangement of the liquid crystal light modulator away fromthe center enabled easier preparation. As a pitch of the liquid crystallattice, the following three patterns were manufactured:

(i) Pitch of liquid crystal lattice: 20 μm (control area: 17 μm, gap: 3μm)

(ii) Pitch of liquid crystal lattice: 20 μm (control area: 18 μm, gap: 2μm)

(iii) Pitch of liquid crystal lattice: 10 μm (control area: 8 μm, gap: 2μm)

As for the orientation of liquid crystals, the orientation direction wasset 45 degrees in case of intensity control, and it was set 0 degree incase of phase control. The liquid crystal spatial light modulator forintensity control and the liquid crystal spatial light modulator forphase control were prepared as a unit with the glass substrate having apattern electrode in common.

FIG. 12 are graphs replaced with drawings showing the optical intensitycontrol characteristic of an optical waveform shaping device. FIG. 12(a) shows a graph of measurement of the optical intensity controlcharacteristic when an ASE light source is used and all the channels arecontrolled collectively. FIG. 12( b) shows a graph of measurement of theoptical intensity control characteristic when all the channels areintermediately controlled. FIG. 12( c) shows a graph of measurement ofthe optical intensity control characteristic when all the channels areOFF controlled. FIG. 12( a) shows that the passbands of adjacent bandsforms continuous passbands in the optical waveform shaping device of thepresent invention. On the other hand, FIG. 12( b) shows that theintensity control variable can be set arbitrarily within the controlrange. FIG. 12( c) shows that output can be suppressed in case of OFFcontrol.

FIG. 13 are graphs replaced with drawings showing frequency spacing ofan optical waveform shaping device. The wavelength of a wavelengthvariable LD light source was swept at a 0.01 nm step, and the power ofeach wavelength with optical intensity controlled was measured with apower meter. The optical intensity control was set for every 1CH, andthe frequency spacing was checked. FIG. 13( a) shows a frequency spacingin case of one ON, FIG. 13( b) shows a frequency spacing in case of twoadjacent ON, FIG. 13( c) shows a frequency spacing in case of three ON,and FIG. 13( d) shows a frequency spacing in case of two separate ON.

FIG. 14 is a graph replaced with a drawing showing the spatialresolution of an optical waveform shaping device. An ASE light sourcewas used and the optical intensity was controlled for every 48CH (ON asa whole; OFF control for every 48CH). The spatial resolution per PAL-SLMcell in each wavelength band was measured from the control wavelengthdifference. As a result, the spatial resolution was 12.1 GHz/cell at awavelength of 1535 nm, the spatial resolution was 10.7 GHz/cell at awavelength of 1550 nm, and the spatial resolution was 9.2 GHz/cell at awavelength of 1565 nm.

FIG. 15 is a schematic diagram showing a device configuration formeasuring an insertion loss. As a result, the insertion loss was 6.5 dBat a wavelength of 1535 nm, the insertion loss was 5.0 dB at awavelength of 1550 nm, and the insertion loss was 7.5 dB at wavelengthof 1565 nm.

Table 1 is a table showing the dispersion characteristic of a grating.

TABLE 1 Wavelength Incident angle Diffraction angle Diffractionefficiency 1540 nm 50 deg 68.9 deg 87.0% 1550 nm 50 deg 70.0 deg 88.2%1560 nm 50 deg 71.2 deg 89.1%

Table 2 is a table showing the diameter of a PAL-SLM incident beam.

TABLE 2 Condensing diameter Condensing diameter Wavelength (x-axis)(y-axis) 1530 nm Φx-40 μm Φx-74 μm 1550 nm Φx-38 μm Φx-65 μm 1570 nmΦx-45 μm Φx-86 μm

INDUSTRIAL APPLICABILITY

The optical waveform shaping device of the present invention ispreferably used in the fields such as optical information andcommunication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing a configuration example of anoptical waveform shaping device of the present invention.

FIG. 2 is a conceptual diagram of a spatial optical modulator having aphase modulation part and an intensity modulation part.

FIG. 3 is a conceptual diagram showing the orientation of an intensitymodulation part and a phase modulation part.

FIG. 4 are conceptual diagrams for explaining polarization control,intensity control, and phase control.

FIG. 5 is a diagram showing an example of an optical waveform shapingdevice which uses a prism as a folded reflector.

FIG. 6 are figures showing a beam incident on cells. FIG. 6( a) shows alight beam on the short wavelength side incident on cells, while FIG. 6(b) shows a light beam on the long wavelength side incident on cells.

FIG. 7 are figures showing an optical waveform shaping device of thepresent invention capable of phase shift compensation. FIG. 7( a) showsan example using an existing driver, while FIG. 7( b) shows an exampleperforming DIO direct control.

FIG. 8 is a comprehensive diagram of the optical waveform shaping deviceaccording to Embodiment 1.

FIG. 9 are schematic diagrams of the optical system according toEmbodiment 1. FIG. 9( a) is a top view, while FIG. 9( b) is a side view.

FIG. 10 is a schematic diagram of a simulation determining the positionof an optical element.

FIG. 11 are figures showing an example of a liquid crystal spatial lightmodulator. FIG. 11( a) through FIG. 11( c) are figures showing anoverview of the actually manufactured spatial light modulator.

FIG. 12 are graphs replaced with drawings showing the optical intensitycontrol characteristic of an optical waveform shaping device. FIG. 12(a) shows a graph of measurement of the optical intensity controlcharacteristic when an ASE light source is used and all the channels arecontrolled collectively. FIG. 12( b) shows a graph of measurement of theoptical intensity control characteristic when all the channels areintermediately controlled. FIG. 12( c) shows a graph of measurement ofthe optical intensity control characteristic when all the channels areOFF controlled

FIG. 13 are graphs replaced with drawings showing frequency spacing ofan optical waveform shaping device. The wavelength of a wavelengthvariable LD light source was swept at a 0.01 nm step, and the power ofeach wavelength with optical intensity controlled was measured with apower meter. The optical intensity control was set for every 1CH, andthe frequency spacing was checked. FIG. 13( a) shows a frequency spacingin case of one ON, FIG. 13( b) shows a frequency spacing in case of twoadjacent ON, FIG. 13( c) shows a frequency spacing in case of three ON,and FIG. 13( d) shows a frequency spacing in case of two separate ON.

FIG. 14 shows a graph replaced with a drawing showing the spatialresolution of an optical waveform shaping device.

FIG. 15 is a schematic diagram showing the device configuration formeasuring an insertion loss.

FIG. 16 is a figure for explaining the optical waveform shaping deviceaccording to the third aspect of the present invention.

FIG. 17 is a figure for explaining the optical waveform shaping deviceaccording to the third aspect of the present invention.

DESCRIPTION OF THE NUMERALS

-   -   11 Branching filter    -   12 Condensing part    -   13 Polarization separation means    -   14 Spatial light modulator

The invention claimed is:
 1. An optical waveform shaping devicecomprising: a polarization separator for polarizing/separating the lightbeam from a light source; a ½ wavelength plate for joining thepolarization planes of a first lightwave and a second lightwavepolarized/separated by the polarization separator; a polarization beamsplitter where the light beams having passed through the ½ wavelengthplate are incident; a Faraday rotator for rotating in a predeterminedamount the polarization planes of the first lightwave and the secondlightwave having passed through the polarization beam splitter; a firstcollimator where the lightwave having passed through the Faraday rotatoris incident; a second collimator where the lightwave having passedthrough the Faraday rotator is incident; a 2-axispolarization-preserving fiber where the lightwaves from the firstcollimator and the second collimator are incident; a third collimatorwhere the light beams having passed through the 2-axispolarization-preserving fiber; a branching filter for branching thelight beam from the third collimator into the light beams of eachfrequency; a condensing lens for condensing the plurality of light beamsbranched by the branching filter; a polarization separation means foradjusting the polarization planes of the light beams having passedthrough the condensing lens; a spatial light modulator having a phasemodulation part and an intensity modulation part where the light beamshaving passed through the polarization separation means are incident,the phase modulation part and the intensity modulation part each havinga plurality of liquid crystal cells in a line or in a matrix existing inthe corresponding spatial positions, the orientation of liquid crystalsof the phase modulation part being parallel to the polarization planeadjusted by the polarization separation means, the orientation of liquidcrystal of the intensity modulation part being 45 degrees offset fromthe orientation of liquid crystals of the phase modulation part; aprism-type folded reflector where the light beams having passed throughthe liquid crystal spatial phase modulation and liquid crystal spatialintensity modulation part are incident; a ½ wavelength plate foradjusting the polarization planes of the lightwaves output from thepolarization beam splitter after having passed through the foldedreflector; and a forth collimator where the light beams having passedthrough the ½ wavelength plate are incident, wherein the light beam fromthe third collimator is frequency separated and is dispersed spatiallyby the branching filter, wherein the spatially dispersed and frequencyseparated light beams are condensed by the condensing lens, wherein thepolarization planes of the condensed light beams are adjusted by thepolarization separation means, wherein the light beams with thepolarization planes adjusted are subjected to either or both ofseparately controlled phase modulation and intensity modulation by thespatial light modulator, wherein the light beams are folded by thefolded reflector, wherein the light beams are condensed through thecondensing lens, wherein the frequency separated light beams aremultiplexed by the branching filter, wherein the lightwave derived fromthe first lightwave is incident on the Faraday rotator through thesecond collimator, wherein the lightwave derived from the secondlightwave is incident on the Faraday rotator through the firstcollimator, wherein the traveling direction of the lightwave derivedfrom the first lightwave and the lightwave derived from the secondlightwave having passed through the Faraday rotator are adjusted by thepolarization beam splitter, wherein the polarization planes of the twolightwaves with the traveling direction adjusted are adjusted by the ½wavelength plate so that the polarization planes are orthogonal to eachother, and wherein the lightwaves with the polarization planes adjustedare output through the forth collimator.
 2. The optical waveform shapingdevice as claimed in claim 1, wherein a plurality of liquid crystalcells of the spatial light modulator constitute one channel with twocells or three cells depending on the diameter of input light.
 3. Theoptical waveform shaping device as claimed in claim 1, wherein theplurality of liquid crystal cells of the spatial light modulatorcomprise a lattice pitch of 10 μm-40 μm.
 4. The optical waveform shapingdevice as claimed in claim 1, wherein the polarization separation meanscomprises a polarization beam splitter and an optical system for guidinga first light and a second light separated by the polarization beamsplitter to the branching filter.
 5. The optical waveform shaping deviceas claimed in claim 4, wherein the third collimator further comprises apolarization module located at the end face of the 2-axispolarization-preserving fiber, the polarization module comprising: afirst optical system for controlling the light traveling to the spatiallight modulator and the first light separated by the polarization beamsplitter after having passed through the spatial light modulator; and asecond optical system for controlling the second light separated by thepolarization beam splitter after having passed through the spatial lightmodulator.
 6. An optical waveform shaping device comprising: apolarization separator for polarizing/separating the light beam from alight source; a branching filter for branching a first lightwave and asecond lightwave separated by the polarization separator into the lightbeams of each frequency; a condensing part for condensing the pluralityof light beams branched by the branching filter; a polarizationseparation means for adjusting the polarization planes of the lightbeams having passed through the condensing part; a spatial lightmodulator having a phase modulation part and an intensity modulationpart where the light beams having passed through the polarizationseparation means; and a prism-type folded reflector where the lightbeams having passed through the spatial light modulator having theliquid crystal spatial phase modulation and liquid crystal spatialintensity modulation part are incident.
 7. The optical waveform shapingdevice as claimed in claim 6, wherein the first lightwave and the secondlightwave reach the branching filter through a first axis and a secondaxis, respectively, of a 2-axis polarization-preserving fiber and alsothe first lightwave and the second lightwave folded through thereflector are output through the second axis and the first axis,respectively, of the 2-axis polarization-preserving fiber.
 8. Theoptical waveform shaping device as claimed in claim 6, wherein aplurality of liquid crystal cells of the spatial light modulatorconstitute one channel with two cells or three cells depending on thediameter of input light.
 9. The optical waveform shaping device asclaimed in claim 6, wherein the plurality of liquid crystal cells of thespatial light modulator comprise a lattice pitch of 10 μm-40 μm.
 10. Theoptical waveform shaping device as claimed in claim 6, wherein thepolarization separation means comprises a polarization beam splitter andan optical system for guiding a first light and a second light separatedby the polarization beam splitter to the branching filter.
 11. Theoptical waveform shaping device as claimed in claim 10, wherein thethird collimator further comprises a polarization module located at theend face of the 2-axis polarization-preserving fiber, the polarizationmodule comprising: a first optical system for controlling the lighttraveling to the spatial light modulator and the first light separatedby the polarization beam splitter after having passed through thespatial light modulator; and a second optical system for controlling thesecond light separated by the polarization beam splitter after havingpassed through the spatial light modulator.